The reaction to form glycolic acid by the carbonylation of formaldehyde with carbon monoxide and water using strong acid catalysts is well known. The basic process was first disclosed by DuPont in U.S. Pat. No. 2,152,852. The process was for the preparation of glycolic acid in the liquid phase by reacting formaldehyde, water and carbon monoxide in the presence of a homogeneous acidic catalyst at temperatures between 50° C. and 350° C. and at a pressure between 5 and 1500 atma. Sulfuric acid, hydrochloric acid, phosphoric acid, boron fluoride, formic acid and glycolic acid are identified as being suitable catalysts.
DuPont went on to obtain further patents in this field, including, U.S. Pat. No. 2,285,448 which related to the hydrogenation of glycolic acid to ethylene glycol, and U.S. Pat. No. 2,443,482 which related to a continuous process for formaldehyde carbonylation.
The process for producing ethylene glycol was commercialised and operated by DuPont until the late 1960's when this route to ethylene glycol became uncompetitive. The plant was then operated for the production of glycolic acid in which sulfuric acid was used as catalyst at a temperature of 200° C. and at a pressure of from 400 to 700 bar.
The processes described in these initial cases suffered from numerous problems. These problems included those attributable to the need to operate at very high pressure. In addition, the selectivity was poor. It was also necessary to contend with the highly corrosive reaction mixture and the difficulty of removing the homogeneous acid catalyst such as sulfuric acid from the reaction product.
Various proposals have been made to address some or all of these problems. For example, U.S. Pat. No. 3,859,349 attempts to address the problems associated with separating the sulphuric acid catalyst and suggests using ion exchange resins as an alternative to neutralisation with calcium carbonate, which had been the previous approach. However, the sulfonic acid based ion exchange resins have limited thermal stability in aqueous environments leading to the loss of acid groups.
Another proposal was that described in U.S. Pat. No. 4,431,486 in which azeotropic distillation of the crude glycolic acid product was proposed as a means of reducing the water content in the recycle to the carbonylation reactors thereby minimising byproduct formation and increasing the yield from the feed formaldehyde.
Another approach has been to look at alternative catalyst systems as a means of reducing the reactor operating pressure. Hydrogen fluoride has been suggested as being a suitable catalyst in U.S. Pat. Nos. 3,911,003, 4,016,208, 4,087,470, 4,136,112 and 4,188,494. Processes which use hydrogen fluoride in place of sulfuric acid as catalyst are suggested to allow operating pressures of 1 to 275 bar.
A further alternative process is disclosed in U.S. Pat. No. 4,052,452 in which Cu(I) or Ag salts in concentrated sulfuric acid are suggested as a means of increasing the carbon monoxide solubility and it is suggested that this enables the operating pressure to be reduced to between 0.1 and 30 atma. Whilst this may address the operating pressure issues, such systems are extremely sensitive to poisoning by water and separation and recycle of the metallic catalyst is difficult.
In U.S. Pat. No. 6,376,723 it is proposed that the reaction should be conducted with an acid catalyst having a pKa value of less than −1 in the presence of a sulfone as a means of moderating the reaction conditions. There is also a suggestion that heterogeneous catalysts could be used.
U.S. Pat. No. 4,140,866 looks at the problems associated with removing the sulfuric acid catalyst from glycolic acid produced by formaldehyde carbonylation. The proposed solution is to first treat the reaction mixture with an alkali metal hydroxide to form the dissolved sulfate salt and this is then precipitated on esterification of the glycolic acid with ethylene glycol and removal of water.
A widely adopted strategy for overcoming the problems associated with separating homogeneous catalysts from reaction mixtures is to replace the homogeneous catalysts with heterogeneous catalysts that can easily be mechanically separated. Several solid acid materials have been suggested as suitable catalysts for formaldehyde carbonylation. These include sulfonic acid ion exchange resins, aluminosilicate zeolites, polyoxometalate salts and alkyl sulfonic acid polysiloxanes.
The use of solid insoluble particulate acidic catalysts having a hydrogen ion exchange capacity in excess of 0.1 milliequivalents per gram was first described in GB1499245. Sulfonic acid based ion-exchange resins, acid clays and zeolites are listed as suitable catalysts. Strongly acidic cation exchange resins in a reaction solvent such as acetic acid are suggested in JP56073042A2 and the use of FZ-1 and ZSM type zeolites in EP0114657.
An alternative process for the preparation of glycolic acid or its esters is disclosed in DE3133353C2. In this process, formaldehyde is reacted with carbon monoxide and water or an alcohol in an inert diluent in two reaction steps. In the first step, formaldehyde is reacted with carbon monoxide using an acidic, solid, insoluble, finely distributed catalyst at a ratio of hydrogen ion exchange capacity of the catalyst to the molar amount of the formaldehyde of 1:1 to 5:1, a temperature of 30° C. to 200° C. and a pressure of 10 to 325 bar. In the second step, water or an alcohol having 1 to 20 carbon atoms is added at a temperature of 20° C. to 200° C. and a pressure of 1 to 325 bar. The catalyst is subsequently mechanically separated from the reaction medium.
KR19950013078B1 relates to a process for producing glycolic acid in which formaldehyde and carbon monoxide are reacted in the presence of water or water-methanol mixture using a heterogeneous solid catalyst, which is polymeric strong acid catalyst ion-exchanged by 5-40 wt % with monovalent metal of Group IB in a water-soluble inert solvent. Dioxane is used as a water-soluble inert solvent.
A similar process is described in KR19950013079B1 in which formaldehyde and carbon monoxide are reacted in the presence of water or water-methanol mixture using a polymeric strong acid catalyst in a water-soluble inert solvent.
A process for continuously manufacturing methyl glycolate from formaldehyde, carbon oxide and methanol is described in KR19950009480B1 in which a flow reactor filled with a polymeric strong acid catalyst is used. Reactant mixture of formaldehyde, water and inert solvent and carbon monoxide is supplied to the upper part of the reactor, and methanol is supplied to the lower part. In the upper part of the reactor, glycolic acid is produced via acid catalysis. In the lower part of the reactor, methyl glycolate is prepared from methanol and formed glycolic acid. The pressure of carbon monoxide is 500 to 6,000 psig and the temperature is 80 to 200° C. The suggested selectivity for this one-step procedure is relatively high.
KR0124821B1 relates to separating methylglycolate from an acidic solution. In this case, the reaction solution formed by a carbonylation reaction and an esterification reaction contains methyl glycolate, dioxane, water, methanol and hydrogen ion. This reaction solution is sent to a neutralization reactor and is neutralized by the addition of alkali to give a salt. The reaction solution containing salt is distilled to separate methanol, water and dioxane from methyl glycolate, salt and dioxane. The methanol separated from dioxane is recirculated to the carbonylation reactor. The solution which separated from the lower part of the distillation tower contains methyl glycolate, salt and dioxane. This is sent to a solid-liquid separator to separate the methyl glycolate from the solvent.
A further process for the production of methyl glycolate is described in KR19950011114B1. In this process formaldehyde is reacted with carbon monoxide to make a glycolic acid. The glycolic acid is then reacted with methanol to make a methyl glycolate. Residual formaldehyde is then reacted with methanol to make methylal. The methyl glycolate and methylal are then separated by distillation. The methylal is reacted with a Fe—Mo catalyst to return it to formaldehyde which is then recovered and concentrated before being recycled.
An alternative heterogeneous acid catalyst for the formaldehyde carbonylation reaction is described in U.S. Pat. No. 6,376,723. Sulfonic acid based ion exchange resins such as Amberlyst 38W and Nafion SAC13 are mentioned as suitable commercially available catalysts. Deloxan ASP 1/9, an alkyl sulfonic acid polysiloxane, is also listed as a suitable catalyst. This material is formed by co-polycondensation of propyl(3-sulfonic acid)siloxane and SiO2.
He et al, in Catalysis Today, 51 (1999), 127-134, describe the use of heteropolyacids as homogeneous catalysts for the condensation of formaldehyde and methyl formate.
A still further process is described in JP2503178. In this process, glycolic acid is formed by hydrolysis of polyglycolide made from formaldehyde and carbon monoxide in the presence of a solid heteropoly acid.
WO2009/140787, WO2009/140788 and WO2009/140850 relate to processes using insoluble polyoxometalate compounds. These compounds either have specific acid properties or are encapsulated within zeolite cages, as solid acid catalysts, to produce glycolic acid from carbon monoxide and formaldehyde. However, the metal salts are prone to leaching of the metal component, which will reduce the number of active acid sites. In the case of zeolite impregnated with polyoxometalate salts, acid leaching will impact both the zeolite substrate and the salts themselves.
There are also a number of cases relating to various substituted organopolysiloxane compounds and their uses. These cases can be grouped into five families which cover different classes of polysiloxane compounds. The five groupings can be typified by: EP1360222B1, EP1786850B1, WO2006/013080A1, WO2007/090676A1 and US2010/0290962A1 which disclose various families of compounds. These documents suggest that these compounds may be useful for carbonylation reactions, but there is no detailed teaching as to how these materials can be used as catalysts for formaldehyde carbonylation in particular or to carbonylation reactions more generally.
It has been suggested that the use of heterogeneous catalysts will reduce the corrosion of the reaction system. None of the heterogeneous catalysts proposed in the prior art has been adopted commercially.
Although there have been numerous patents and publications relating to the production of ethylene glycol from glycolic acid which is formed by carbonylation of formaldehyde, there remains a need for an improved process which can compete economically with the established industrial production route.
The various approaches to trying to solve the problems associated with the reaction can be summarised into two categories. The first relates to the investigation of homogeneous catalyst systems which operate at lower pressure and lower acid concentration than has previously been achievable.
The second relates to the investigation of heterogeneous solid acid catalysts as these benefit from easier separation of the catalyst and reduced reactor corrosion. However, the solid catalysts proposed to date have also proved to have a number of shortcomings and have not been adopted commercially. These catalysts generally lack the thermal and chemical stability required to withstand the severe reaction conditions.
For example, aluminosilicate zeolites are not stable under highly acidic conditions, as the aluminium is leached from the structure causing it to collapse. This results in loss of activity and eventually complete disintegration of the catalyst (Pan et al, 1994, Studies in Surface Science and Catalysis). With a view to avoid this problem, it is proposed in EP0114657 that the reaction should be operated such that the amount of acid formed is limited, but this reduces the efficiency of the reactor and exacerbates separation problems.
It is well known that sulfonic acid based ion exchange resins have limited thermal stability in aqueous environments leading to a loss of acid groups. Furthermore it has been found that formaldehyde attacks the aromatic rings within styrene/di vinyl benzene based resins causing swelling and further loss of acid groups.
There has also been a suggestion that substituted organopolysiloxane compounds, such as Deloxan ASP 1/9, Quadrasil—SA and Silicycle (SCX-2), and alkyl sulfonic acid polysiloxanes, can be used but these have been found to quickly lose catalytic performance at effective process conditions. This has been attributed to the loss of the tethered organic acid groups due to hydrolysis.